Paligorskite dielectric capacitor



May 25, 1965 L. J. HOGUE PALIGORSKITE DIELECTRIC CAPACITOR Filed May 26, 1959 Meta/ [/ectrade-\ United States Patent 3,185,906 PALIGORSKITE DIELECTRIC CAPACITOR Lawrence J. Hogue, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed May 26, 1959, Ser. No. 815,786 1 Claim. (Cl. 317-458) This invention relates to high temperature dielectric materials and to electrical devices embodying such materials. More particularly, the invention relates to a high temperature dielectric material made from reconstituted paligorskite (palygorskite) and to electrical devices embodying such material.

The use of sheet-like material such as kraft paper, manila paper and sheets reconstituted from asbestos and the like as dielectric spacers and insulators for electrical equipment is well known. At temperatures below about 80 C., the use of paper such as kraft paper and manila paper results in a device which will not deteriorate over a rather extended period of time. However, at temperatures of above about 120 C. or higher, kraft paper, manila paper and similar materials deteriorate rather rapidly, reducing eventually to a carbonaceous material and resulting in the failure of the device. Asbestos sheets are suitable for use at elevated temperatures as insulating materials. However, asbestos is characterized by an undesirably high power factor which cannot be tolerated in many applications. Likewise, the combination of asbestos fiber with other materials, such as bentonite or other bonding clays, results in a material which also has an undesirably high power factor.

A principal object of this invention is to provide an insulating or dielectric material, and electrical devices embodying such insulating material, which can readily be made and which will withstand elevated temperatures without excessive electrical losses or physical deterioration.

The invention will be better understood from the fol lowing description taken in conjunction with the accompanying drawing which illustrates the use of the dielectric in combination with metal electrodes to form a capacitor structure.

Briefly stated, the present invention constitutes an electrical insulation and devices embodying such insulation which comprises reconstituted paligorskite or paligorskite in sheet or paper form. It was unexpectedly found that sheets of reconstituted paligorskite as opposed to other materials of this general type, such as asbestos, displayed very low electrical losses at elevated temperatures and over long period of time.

Paligorskite is a fibrous silicate material, believed to be a hydrous magnesium aluminum silicate, and is somewhat similar in chemical composition to asbestos. It occurs in nature in various forms, generally constituted of fibers which are known variously as mountain cork, mountain leather, and mountain Wood. The natural material is mixed to a somewhat greater or lesser degree with impurities such as twigs, dirt,'and the like, or other foreign material. It has been found that paligorskite can be prepared in the form of a pulp or slurry in a suitable dispersing agent, such as water and the like, which may then be laid down in normal paper-making fashion to form sheets of reconstituted paligorskite, which can efficaciously be used as electrical insulation at elevated temperatures.

Any concentration of paligorskite in the water slurry up to about 2 /2 percent by weight of paligorskite to water may be prepared. At concentrations of slurry Patented May 25, 1965 ICC higher than about 2 /2 percent (by weight) the pulp becomes too thick to be easily dispersed as a water suspension. It has been found convenient in a typical case to make a slurry consisting of 3.75 grams of paligorskite, 500 cc. of water, along with 0.1 percent benzoic acid based on the weight of the paligorskite, to prevent the formation of mold over a period of time, the whole being blended or stirred as in a high-speed blender, such as a Waring Blendor, or in a paper beater machine, for about 10 minutes, or until a smoothpulp is obtained. The paligorskite slurry takes on the characteristics of a welldispersed thick and gelatinous material. It has been found that the addition of about 1 cc. of a wetting agent, such as Aerosol OT makes for a final sheet material which is more even in texture, contains fewer air pockets, and is in general smoother and less lumpy in the pulp stage than without the wetting agent. This stock pulp (0.75% paligorskite) itself may be laid down as a sheet or it may be diluted. It has been found convenient to prepare a suspension consisting of 50 cc. of the stock pulp along with about 450 cc. of water which is thoroughly mixed together and then laid down to form a sheet on a metal screen or a porous cloth in, a manner similar to the art of paper making. A typical sheet prepared in this manner averages from about 0.003 in. to about 0.005 in. in thickness. The final sheet material has a feel somewhat similar to that of kid leather, but is rather more readily scuffed when rubbed or abraded. Papers or sheets of varying thickness may, of course, be prepared by controlling the amount of paligorskite laid down per unit area. Sheets laid down from the paligorskite material as received have a color ranging from white to tan to brown. This color variation of the native paligorskite is probably due to various impurities, and a white or nearly white material which is improved in its electrical characteristics may be prepared by bleaching the paligorskite before preparation of the paper. Such bleaching may be carried out by the use of relatively strong acids, such as hydrochloric acid, sulfuric acid, oxalic acid, nitric acid, and acetic acid, which are added to the pulp as prepared and the material heated to a temperature of about C. to C. for periods up to about one hour until the pulp turns a whitish color or becomes white after water washing. Concentrated hydrochloric acid is preferred since itlappears 'to wash out of the pulp more readily with water, it being necessary in every case of course to remove the acid as by washing. Good results have been obtained by adding to the original slurry or pulp suspension about 20 cc. of 85% phosphoric acid, about cc. of concentrated hydrochloric acid, about 20 grams of oxalic acid, about 40 cc. of concentrated nitric acid, or 20 cc. of sulfuric acid, heating the mixture for the above period of time and then thoroughly washing the acid out with water. It will be realized that larger or smaller proportions of the acid may be used, depending upon the time and temperature of treatment.

It has also been found that the final material is improved, insofar as electrical characteristic are concerned, by heating the paligorskite as received and, after the removal of gross foreign material, for about 20 hours at about 300 C. to 500 0., preferably 400 C. If desired, this heat treatment may be delayed until the formation of the sheet or paper with similar salutary results.

The following examples will illustrate the practice and advantages of the present invention, it being realized that such examples are exemplary only of the invention and not limiting in any way, except as specifically set 7 3 forth in the claim appended hereto. Test stamp capacitors were prepared using 1 /2 X 2% in. aluminum foil sheets, about .003 in. in thickness, and having attached thereto contact ears. Between the foils there were placed sheets of the dielectric material or insulation to be tested and the outside of the aluminum foil sheets was enclosed by glass plates in the usual manner. The dimensions of the glass plate and the insulation material were about 2% in. X 3%- in. The capacitors so prepared were vacuum impregnated in the usual manner, with various liquid impregnants, including capacitor mineral oil and synthetic dielectrics, such as pentachlordiphenyl, as well as silicone oil and other materials.

Capacitors impregnated with mineral oil and pentachlordiphenyl were tested for power factor at 25 C. and 75 C. with the results shown in Table I. Capacitors having as a dielectric sheet insulator other materials are also shown in the table, including kraft paper and manila paper, asbestos, and an asbestos-bentonite clay sheet.

From Table II, it will be se n that the dielectric strength of the paligoskite materials is suitable high and particularly so with the material which has been bleached, and heat-treated, as described, such material being equivalent to kraft paper and, in addition, not being deteriorated by exposure to elevated temperatures. Once again, it shown that the present materials are far and away better than asbestos, which is another, much-used, in organic, high-temperature resistant, electrical insulating material.

Shown in Table III is the room temperature dielectric strength of paligoskite which has been bleached and heat-treated as above, as compared with kraft paper, manila paper, and asbestos, the test being carried out with a mineral oil impregnant.

Table III Table I Total Dielectric thickness, strength, Power factor (percent) 1115 Vous/m1 h t Magcrial 3 6 3 200 s ccsan thickne \l cal il t ll d h 1 1 SS) 1 m r 0 Pen am or ID my Paligorskite (bleached and heat treated)... g i, at

a "-0 ,-o u i .0 "20 25 20 C 75 Kralt paper 3 i' Manila paper.. 10. O 1, 420 Kraft: Asbestos 9. 6 1, 200

(8 X .00052") 29 S3 35 1. 11 (2 x .0040) 40 1.38 45 1. 29 30 Manila (2 x .003") 32 1.11 .33 3.78 Asbestos-bentonite 2 11.0035) 14.2 19.1 12.7 20.2 I e l g i 3- 6 2- Once again, it IS quite evident that the present ma- 3,131,3 9; ig t ggggjf: 2 :3 :2 $13 terials compare very favorably with usual high dielectric P s gggg W -w 36 1 47 r 2 breakdown kraft paper and manila paper, and at the same time have resistance to elevated temperatures, which It will be noted that the paligorskite insulating materials have by far a much lower power factor at 25 C. and 75 C., than other heat resistant materials, such as asbestos and combination of asbestos with other materials. Furthermore, the power factor of the paligorskite not only compares favorably with that of kraft paper and manila paper, but it will be realized that at temperatures above about 120 C., the kraft and manila papers rapidly deteriorate, whereas the paligorskite, being entirely inorganic, will withstand temperatures of over 500 C. without mechanical deterioration. Attempts to obtain lower power factors with asbestos by refining, as by acid treatment or water washing did not result in any appreciable reduction of the power factor.

The dielectric strength of the present materials is likewise very desirable, especially as compared to typical materials used in similar applications. Shown in Table 11 below is the dielectric strength of the present paligorskite material as untreated and after being heated for 20 hours at 400 C., and after a phosphoric acid bleach as described above, plus heat treatment for 24 hours at 400 (3., all compared with materials such as kraft paper, manila paper, asbestos, and asbestos-bentonite compositions. The potential was rapidily applied in air.

is not shown by these materials.

Shown in Table IV below is the room temperature dielectric strength of the present materials with a pentachlordiphenyl impregnant.

From the above, it will be evident that the present materials have a suitably high dielectric breakdown when used with synthetic impregnating dielectric materials.

The present materials have also been used in conjunction with a typical dimethylpolysiloxane oil having at 25 C. a power factor of 0.29% and at C. a power factor of 0.58%

Shown in Table V below are high temperature power factors and megohm-microfarad product values for various capacitors having natural and synthetic mica and paligorskite dielectrics. The synthetic mica used was a fluorphlogopite and all of the mica capacitors contained three dielectric sheets mounted between metal foils. The paligorskite capacitors were in the first two instances single sheets of dielectric material interleaved with metal foil while the last two samples were single sheets having the metal armature evaporated thereon. Those materials treated with silicone resin were coated and impregnated r O with the resin. The thickness of the sheets is given in the table.

Table V Tempcra Power Megohm- Material ture, 0. factor micro- (perccnt) farad Synthetic mica, paper (0.0045") (sili- 150 2. 3 317 cone treated) 250 3. 9 37 350 18.4 3. 9 Phlogopite mica, paper (0.0037") (sili- 150 3. 131

cone treated) 250 6.5 3.6

350 15. 9 .37 Muscovite mica, paper (0.0040") (sili- 150 1. 2 272 cone treated) 250 3. 0 4. 4

350 13. 1 33 Paligorskite, washed (0.0041") (sili- 150 6.2 61

cone treated) 250 5.8 5. 4

350 11.8 .39 Paligorskite, H01 bleached (0.0048) 150 4.1 61 (silicone treated) 250 3. 3 6

350 8.7 .52 Paligorskite, washed (0.0024") (un- 150 6.1 145 treated) 250 4. 0 113 350 7.0 88 Paligorskite, H01 bleached (0.0032") 150 4.7 111 (untreated) 250 2. 4 63 350 5.6 25

From the above it will be quite apparent that the present materials are very efficacious for use at high temperatures, both as regards their power factor and their megohrn-microfarad characteristics.

The present materials may be mixed with various other dielectric materials with no essential degradation of the power factor or other dielectric characteristics. Such material is typically used in amounts up to that of the paligorskite in the original pulp-forming suspension. For example, magnesium oxide, titanium oxide, lignin, aluminum hydroxide, sulfur, and ammonium borate have been used in amounts equal to the paligorskite and have produced materials having about the same power factor as the paligorskite Without the additive material.

It will be seen, then, that the present paligorskite sheet materials are useful in electrical applications where such sheet-like material is indicated, and particularly where subjection to high temperatures occurs. The present materials have also been combined with the usual resinous materials to form bonded sheets and have been molded into various shapes. The present dielectrics are useful not only in capacitors but as cable insulation or slot insulation, for panelboard insulation, arc chutes, and the like. They may be used in connection with impregnants or alone as indicated.

What I claim as new and desire to secure by Letters Patent of the United States is:

A capacitor comprising armatures, and as a dielectric spacing therefor, a sheet material comprising heat treated reconstituted paligorskite having improved power factor, dielectric strength, and niicrotarad product values, said heat treatment being carried out at a temperature of at least 300 C.

References Cited by the Examiner UNITED STATES PATENTS 2,614,05 5 10/52 Senarclens 1623 2,616,801 11/52 Badollet 162-453 2,804,908 9/57 Spooner 156-319 FOREIGN PATENTS 9/ 52 Great Britain.

OTHER REFERENCES LARAMIE E. ASKIN, Primary Examiner.

LLOYD MCCOLLUM, SAMUEL BERNSTEIN, JOHN P. WILDMAN, E. JAMES SAX, JOHN F. BURNS,

Examiners. 

